This invention relates to generally lighting, and more particularly, to ceramic discharge chambers for a lamp, such as a ceramic metal halide lamp or a high pressure sodium discharge lamp.
The present invention relates generally to lighting, and more specifically, to a ceramic arc chamber for a discharge lamp, such as a ceramic metal halide lamp. This invention relates particularly to a method of manufacturing ceramic arc chambers, and more particularly, to a method for sintering ceramic arc chambers.
Discharge lamps produce light by ionizing a fill such as a mixture of metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill, also known as a xe2x80x9cdosexe2x80x9d, emits a desired spectral energy distribution in response to being excited by the electric arc.
Initially, the discharge chamber in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into a desired chamber geometry after being heated to a softened state. Fused quartz, however, has certain disadvantages which arise from its reactive properties at high operating temperatures. For example, at temperatures greater than about 950 to 1,000xc2x0 C., the halide fill reacts with the glass to produce silicates and silicon halide, reducing the fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall. These fill depletions cause color shift over time, which reduces the useful life of the lamp.
Ceramic discharge chambers were developed to operate at high temperatures for improved color temperatures, color renderings, luminous efficacies, while significantly reducing reactions with the fill material. U.S. Pat. Nos. 4,285,732 and 5,725,827, for example, disclose translucent polycrystalline sintered bodies where visible wavelength radiation is sufficiently able to pass through to make the body useful for use as an arc tube.
Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder and then sintered together. For example, referring now to European Patent Application No. 0587238, five ceramic parts are used to construct the discharge chamber of a metal halide lamp. Two end plugs with a central bore are fabricated by die pressing a mixture of a ceramic powder and inorganic binder. A central cylinder and the two legs are produced by extruding a ceramic powder/binder mixture through a die. After forming the part, it is typically air sintered between 900-1400xc2x0 C. to remove organic processing aids. Assembly of the discharge chamber requires tacking of the legs to the cylinder plugs, and the end plugs into the end of the central cylinder. This assembly is then sintered to form joins which are bonded by controlled shrinkage of the individual parts.
In alternative structures, two and three component lamps have been developed and include end pieces of tubes/end caps and a central body. Typically, to facilitate the appropriate binding and mating of these components, the components are horizontally aligned within a molybdenum sintering tube. The sintering process performed in molybdenum tubes is a relatively expensive operation. More particularly, a 3xe2x80x2 molybdenum tube costs several hundred dollars per piece. Accordingly, it would be desirable to provide an alternative housing within which a ceramic discharge chamber can be sintered.
According to an exemplary embodiment of this invention, a method is provided for making a ceramic discharge chamber by forming a plurality of chamber components and assembling the chamber components into a chamber preform. The chamber preform is then positioned within a tube, the tube being comprised of an alumina cermet. The tube containing the chamber preform is then sintered.
This exemplary embodiment of the invention advantageously reduces the unit cost of manufacture of the ceramic discharge chambers. More particularly, the alumina cermet tubes of the present invention are relatively inexpensive yet exhibit long operational life. Furthermore, the alumina cermet tubes reduce the formation of blemishes in the arc discharge chambers which may occur as a result of a seam which is present in a traditional molybdenum tube.